This application claims the priority of German Patent Application No. 198 18 375.5, filed Apr. 24, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a PTCR resistor. The abbreviation PTCR stands for Positive Temperature Coefficient of Resistance. These are therefore resistors which, at least within a limited temperature interval, have a positive temperature coefficient.
Electric resistors for higher powers are particularly made of metallic alloys. The electric resistance of a cylindrical wire or of a parallelepiped body can be calculated according to the following formula ##EQU1## wherein the variable .rho. signifies the specific resistance or resistivity; l signifies the length of the resistance body; and A signifies its cross-sectional surface. Pure metals have the lowest specific resistance. With a specific resistance of 0.016.times.10.sup.-6 .OMEGA.m, silver is the material with the best electric conductivity at room temperature. Higher specific resistances are found in the case of (1) copper-nickel-manganese alloys (for example, in the case of CONSTATAN, with .rho.=0.5.times.10.sup.-6 .OMEGA.m or in the case of MANGANIN, with .rho.=0.43.times.10.sup.-6 .OMEGA.m); or (2) chromium-nickel alloys, about .rho.=1.times.10.sup.-6 .OMEGA.m.
If resistance components with a power below one watt, as they are typically used in the case of printed circuit boards, are disregarded, low electric resistance values are achieved according to Equation (1) by means of wide, short bodies of a very conductive material. Electric resistors with a high resistance value consist of long, thin wires, which are therefore often manufactured in a wound form, and consist of a poorly conducting material.
Because of the length of the electric resistance wire, such wound resistance components have an inductance L which should not be neglected. This inductance L becomes particularly noticeable when high currents flow through the component. In order not to destroy the wire by heating, large cross-sections A and therefore large wire lengths l are required which lead to a high inductance. Specifically, high currents which flow through such resistance components must often be controlled. Typically, such high currents are timed at a variable pulse duration ratio. If alternating currents are involved, a phase angle control or zero voltage control is used. In these cases, the inductance of the component must be as low as possible in order to avoid induction-caused voltage excesses. Thus, for high-current applications, resistance materials of an appropriately high specific electric resistance are required to ensure that the wire length and therefore the inductance are within reasonable limits for large cross-sectional surfaces.
Simultaneously, a positive temperature coefficient of resistance a is required for many applications, wherein ##EQU2## or, relative to the specific resistance, ##EQU3##
Resistance components with a positive temperature coefficient of resistance increase their resistance value as the component temperature rises. Thus, a certain self-limiting of the heating current occurs. Typical temperature coefficients of metals and metallic alloys are between 0 and 8,000.times.10.sup.-6 /K.
Higher specific resistances are found in the case of intermetallic alloys of Main Group IV of the Classification of Elements, such as silicon or germanium. However, as a rule, these have a negative temperature coefficient. Although the specific resistance can be changed within wide ranges by doping, a positive temperature coefficient of resistance exists only within a narrowly limited temperature interval (i.e., the so-called range of impurity exhaustion). In addition, silicon technology requires high expenditures. Large shaped bodies which consist of monocrystals and are provided with a doping agent are very expensive and can be produced in only a few simple geometrical shapes.
In contrast, ceramics can be produced at significantly lower cost and in almost any geometrical shape. Mainly special PTCR ceramics are used in electrical engineering. At a defined temperature T.sub.s, PTCR ceramics have an abrupt rise of the electric resistance, as schematically illustrated in FIG. 1. Such ceramics can be used, for example, as a completely self-regulating component in the form of (1) a heating resistor in hot air fans, heaters and similar devices; (2) a current-limiting element for degaussing coils of picture tubes; or (3) a temperature sensor with an on-off characteristic. As an example, FIG. 1 shows the electric resistance as a function of the temperature of three different component parts. A typical transition temperature T.sub.s, at which the temperature coefficient a may assume values of up to 100%/K, is characteristic of each component part.
This so-called ceramic PTCR effect is attributable to the phenomenon that above a defined temperature, the grain boundaries of the ceramic material become so highly resistive that they determine the overall resistance of the ceramic material and thus of the component part. In this case, the whole electric power falls on the thin highly resistive grain boundary layer of the ceramic material, which may lead to considerable thermal tensions and to the destruction of the component part particularly when large currents are switched.
The materials used for such PTCR ceramics are barium titanate ceramics (BaTiO.sub.3) which are made electrically conductive with donor dopings in the per mil range. (See Russian patent document SU 81 77 58 B; Japanese patent document JP 01 14 3201 A.) Slight acceptor dopings, which are allocated to the grain boundaries, further increase the temperature coefficient (German Patent Document DE 27 53 766 A1). The extremely steep rise of the resistance takes place approximately at the Curie temperature of the material.
At the Curie temperature, the lattice changes from the ferro-electric phase to the para-electric phase. The transition temperature T.sub.s is therefore virtually identical with the Curie temperature and is therefore a true characteristic of the material. Curve 2 in FIG. 1 describes the temperature variation of the electric resistance of the material BaTiO.sub.3. By replacing the barium (Ba) with strontium (Sr), the Curie temperature and thus also T.sub.s changes toward lower temperatures (curve 1 in FIG. 1). A change of T.sub.s toward higher temperatures is achieved by means of lead (Pb) as a barium substitute (curve 3 in FIG. 1). This PTCR effect is discussed in detail, for example, in (1) Huybrechts B. et al., Review: The Positive Temperature Coefficient of Resistivity in Barium Titanate, J. Mat. Sci., 30, 2463-74 (1995); (2) Daniels J. et al., The PTC Effect of Barium Titanate, Phil. Tech. Rev., 38, 73-82 (1978); and (3) Heywang, W., Resistivity Anomaly in Doped Barium Titanate, J. Am. Cer. Soc. 47 (10) 484-489.
Negative temperature coefficient of resistance (NTCR) behavior (.alpha.&lt;0) exists above and below the very narrow PTCR range, which is not tolerable for all applications. The large temperature coefficient of the resistance of up to 100%/K is also intolerable for some applications. The danger of destruction by thermally induced mechanical tensions during the shifting of large currents, a disadvantage which is decisive for some applications, results from the fact that the whole electric power falls at the grain boundaries and cannot be avoided.
It is therefore an object of the present invention to provide a PTCR resistor by means of which the above-mentioned disadvantages of the state of the art can be avoided.
This object is achieved by a para-electric PTCR resistor according to the present invention having the following composition: EQU [(Ba.sub.1-e-x Sr.sub.x R.sub.e).sub.1-a M.sub.a ][(Ti.sub.1-b-d N.sub.b Q.sub.d)].sub.c O.sub.3,
wherein M is a trivalent or quadrivalent element or a mixture of two or several of these elements; N is a quintavalent or hexavalent element or a mixture of two or several of these elements; Q is a quadrivalent element or a mixture of two or several of these elements; R is a bivalent element or a mixture of two or several of these elements; and
0.5.ltoreq.x.ltoreq.1, particularly 0.6.ltoreq.x.ltoreq.1, PA1 a=0 or 0.0003.ltoreq.a.ltoreq.0.3, PA1 b=0 or 0.0003.ltoreq.b.ltoreq.0.3, PA1 0.8.ltoreq.c.ltoreq.1.2, PA1 0.ltoreq.d.ltoreq.0.2, PA1 0.ltoreq.e.ltoreq.0.2, PA1 wherein if a is 0, then b cannot be zero.
This ceramic material for temperature-dependent resistance components based on doped titanates has a continuous positive temperature coefficient of resistance in the temperature range from -50.degree. C. to 250.degree. C. In this case, values of up to 9,500.times.10.sup.-6 /K are reached at room temperature. The specific resistance at room temperature can easily be adjusted between approximately 10.sup.-6 .OMEGA.m and approximately 3.times.10.sup.-1 .OMEGA.m. The Curie temperature of the material according to the present invention is far below the above-mentioned temperature range of -50.degree. C. to 250.degree. C. The specific resistance is therefore composed only of the volume resistance, so that the total electric power falls in the grain volume and not at the grain boundaries. As a result, a homogeneous temperature distribution and therefore a good stressing capacity of resistors with respect to power are provided.
An example of a composition of a ceramic titanate material according to the present invention is as follows: EQU (Ba.sub.1-x Sr.sub.x).sub.1-a M.sub.a (Ti.sub.1-b N.sub.b).sub.c O.sub.3(4)
wherein Ba, Sr, Ti and O are the chemical elements barium, strontium, titanium and oxygen; M is a lanthanide (Atomic Numbers 57 to 71 in the Classification of Elements) or yttrium (Y), indium (In) or thallium (Tl) or a mixture of two or several of these elements. Since the element M is trivalent or quadrivalent and replaces a bivalent element (barium or strontium), it may physically be considered a donor. N is a quintavalent or hexavalent element which can be a substitute for titanium, for example, phosphorus (P), vanadium (V), chromium (Cr), manganese (Mn), arsenic (As), niobium (Nb), antimony (Sb), tantalum (Ta), molybdenum (Mo), tellurium (Te) or tungsten (W) or a mixture of two or several of these elements. Since the element N is quintavalent or hexavalent and replaces a quadrivalent element (titanium), it can physically also be considered a donor. The variables a and b indicate the donor content.
This compound is usually present in a perovskite structure (so-called ABO.sub.3 -structure). In order to control the sintering activity, the A/B ratio can be varied as shown in Jonker and Havinga, The Influences of Foreign Ions on the Crystal Lattice of Barium Titanate, Mat. Res. Bul. 9, 147-156 (1974). The following applies in the above-mentioned example: A=(Ba.sub.1-x Sr.sub.x).sub.1-a M.sub.a and B=Ti.sub.1-b N.sub.b. The parameter c in Equation 4 characterizes the A/B ratio. In the special case when c=1, a second-phase-free cubic perovskite structure is present.
A portion of the quadrivalent titanium element positioned in the B-position may also be substituted by other quadrivalent metal cations, such as zirconium (Zr), tin (Sn) or similar cations, without any basic changes of the electric characteristics of the material according to the present invention. An analogous situation applies to the substitution of a portion of the bivalent barium (Ba) or strontium (Sr) by other bivalent metal cations, such as calcium (Ca), magnesium (Mg) or lead (Pb)
According to the present invention, the strontium portion with 0.5.ltoreq.x.ltoreq.1 in Equation 4 is selected such that, at application temperatures around room temperature, which are typical for electric components, the ceramic titanium material is always in the para-electric condition far above Curie temperature. A grain boundary phenomena which leads to a resistance anomaly, as described in the case of the typical PTCR effect, does not occur.